The present invention relates to electrically conductive brushes for dynamoelectric machines and, more particularly, to apparatus for holding brushes in a dynamoelectric machine for conducting electricity between stationary and rotary elements thereof.
Although the present invention may have general applicability to other large dynamoelectric machines, for concreteness of description, a large alternating current generator provides the exemplary environment.
A large alternating current generator employs a cylindrical rotor rotating within a stator for generating electric energy. Conventionally, the rotor is a field member and the stator is an armature. The rotor is excited by a source of direct current to produce two or more electro-magnetic poles which, when rotated within the armature, induce electric current and voltage in the armature coils. The induced current is delivered to an external load.
A substantial current is required to excite the rotor field. The excitation current is delivered to collector rings on the rotor from an external direct current generator through a plurality of carbon brushes. The brushes are held in stable contact with the collector rings by brush holders.
U.S. Pat. No. 3,864,803, and the U.S. patents referenced therein, disclose numerous devices for holding a carbon brush in contact with a collector ring. In particular, the '803 patent discloses a rectangular brush box into which a rectangular carbon brush is slidably inserted. A spring urges the brush toward the collector ring. A clamping arrangement bears against the side of the brush to hold the brush in its retracted position within the holding fixture while inserting it into a brush box. The clamping arrangement is released automatically as the holding fixture is installed in the brush box, thus permitting the spring to displace the brush into contact with the collector ring.
For maximum brush stability, it is desirable that the brush box extend as close to the collector ring as possible. In the prior art, the brush box extends to within about 1/8 inch of the collector ring.
As the power ratings of alternating current generators grow larger, the number of required brushes disposed about each collector ring can exceed 100. The individual brush holders of the above-referenced patent consume so much space in the vicinity of the collector rings, it is impossible to install enough brushes about the perimeter of the collector rings without lengthening the collector rings, or finding an improved method of packing more brushes in a smaller space. This problem is solved in U.S. Pat. No. 3,387,155 wherein several brush boxes are formed into a ganged brush holder for securing, for example, four, six or more brushes side by side. The clamping mechanism provides fingers for bearing on the sides of the brushes until released during installation in the machine.
The device of this latter patent extends the ganged brush boxes to about 1/8 inch from the collector ring. The temperature rise experienced by brushes is most extreme in the vicinity of the interface between the brushes and the collector ring. The brush boxes prevent free circulation of air in this critical neighborhood, thus limiting the current which ganged brushes can conduct.
In a ganged brush holder, it is difficult to produce a clamping mechanism capable of applying equal force to the sides of all brushes. Thus, a ganged brush holder may fail to retain one of more of the brushes in their retracted positions for installation in the machine. As a possible solution, a plate may be hinged under the working ends of all of the brushes to hold them in the retracted position. The plate is displaced out of contact with the brushes during installation, thereby releasing the brushes to move into contact with the collector ring. This solution requires that the ends of the brush boxes be spaced further away from the collector rings to leave space for the plate between the ends of the brush boxes and the collector ring. Thus, the ability of the brush boxes to provide stable support in the vicinity of the collector rings is reduced.
U.S. Pat. No. 4,329,611 discloses an inside-out brush holder for a single brush which omits the brush box and substitutes a pair of cylindrical brush pins fitting into cylindrical holes in the brush. A constant-force spring urges the brush along the pins toward the collector ring. A clamping mechanism functions in a manner analogous to that in the box-type holder. Since the box is omitted, substantially free air flow about the exterior of the brush is effective to cool the brush, particularly in the critical region in the vicinity of the interface with the collector ring. This patent also suggests, without disclosure of a structure to accomplish it, that the inside-out brush holder may be applied to the ganged brush holder of the U.S. Pat. No. 3,387,155.
The need has now arisen for specific apparatus for a ganged, inside-out brushholder.